Methods of heating a steel workpiece include indirect heating and direct heating. An example of the indirect heating is furnace heating. Examples of the direct heating include induction heating in which eddy current is applied to a workpiece to heat the workpiece and direct resistance heating in which electric current is directly applied to a workpiece to heat the workpiece.
JP 3587501 B2 discloses a method of heating a plate workpiece by direct resistance heating, the workpiece having a heating area with a varying cross-section in which the thickness or the width varies in the longitudinal direction. The heating area of a workpiece is sectioned into a plurality of strip-shaped segments along the longitudinal direction of the workpiece, a pair of electrodes is provided for each segment, and electric current is supplied to each pair of electrodes.
JP 2013-114942 A discloses a method of heating a plate workpiece by direct resistance heating, the workpiece having a heating area with a varying cross-section. For example, in the heating area of a workpiece in which the width monotonically decreases from one end to the other end in the longitudinal direction, a pair of electrodes is disposed at one end having a relatively large width, one electrode moves along the longitudinal direction while supplying a constant current between the pair of electrodes, and the moving speed of the electrode is adjusted base on the variation in the width of the workpiece. In the heating method disclosed in JP 3587501 B2, since multiple pairs of electrodes are required for a single heating area and the electric current is adjusted for each pair of electrodes, the configuration of the heating apparatus is complicated. On the other hand, in the heating method disclosed in JP 2013-114942 A, since a heating area can be heated with a single pair of electrodes, it is possible to simplify the configuration of the heating apparatus.
However, in the heating method disclosed in JP 2013-114942 A, the electric current flowing between the pair of electrodes is kept constant and the moving speed of the electrode is adjusted based on the variation in the width of the workpiece. In order to heat the heating area of the workpiece, for example, at a uniform temperature using this heating method, it is necessary to enhance responsiveness of the moving electrode to speed control. However, since the moving of the electrode is accompanied by moving of a support member of the electrode, a relatively heavy object is moved. Accordingly, in order to ensure the responsiveness of the moving electrode to speed control, an output corresponding to a drive source is required and relatively advanced control is necessary.